Nuclear medicine examination represented by positron emission tomography (hereinafter referred to as PET) and single photon emission computed tomography (hereinafter referred to as SPECT), is effective in diagnosing a variety of diseases including heart disease and cancer. These techniques involve administering an agent labeled with a specific radioisotope (hereinafter referred to as radiopharmaceutical), followed by detecting γ-rays emitted directly or indirectly from the agent. Nuclear medicine examination is characteristic in that it has not only such superior performances as high specificity and sensitivity to diseases, but also an advantage of providing information on the functionality of lesions, compared to other examination techniques.
For example, [18F]2-fluoro-2-deoxy-D-glucose (hereinafter referred to as “18F-FDG”), one of radiopharmaceuticals used for PET examination, tends to be concentrated in area where glucose metabolism is enhanced, thereby making it possible to specifically detect tumors in which glucose metabolism is enhanced.
Nuclear medicine examination is performed by tracing a distribution of an administered radiopharmaceutical, and data obtained therefrom vary depending on nature of the radiopharmaceutical. Thus, different radiopharmaceuticals have been developed for different diseases, and some of them are put into clinical use. There have been developed, for example, various tumor diagnostic agents, bloodstream diagnostic agents and receptor mapping agents.
In recent years, a series of radioactive halogen-labeled amino acid compounds including [18F]1-amino-3-fluorocyclobutanecarboxylic acid (hereinafter referred to as [18F]FACBC) have been designed as novel radiopharmaceuticals, and their clinical application is under examination (Patent Document 1, and non-Patent Documents 1 and 2). [18F]FACBC is considered to be effective as a diagnostic agent for highly proliferative tumors, because it has a property of being taken up specifically by amino acid transporter.
As processes for producing [18F]FACBC, there are disclosed processes which include: providing 1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)-sulfonyl)oxy]-cyclobutane-1-carboxylic acid ester as a labeling precursor, substituting the triflate group at position 3 of the precursor with radioactive fluorine, and carrying out deprotection by subjecting the resulting compound to an acidic condition (Patent Document 1, and non-Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Laid-open No. 2000-500442.    Non-Patent Document 1: Jonathan McConathy et al., “Improved synthesis of anti-[18F]FACBC: improved preparation of labeling precursor and automated radiosynthesis.”, Applied Radiation and Isotopes, (Netherlands), 2003, 58, p. 657-666.    Non-Patent Document 2: Timothy M. Shoup et al., “Synthesis and Evaluation of [18F]1-Amino-3-fluorocyclobutane-1-carboxylic Acid to Image Brain Tumors.”, The Journal of Nuclear Medicine, 1999, 40, p. 331-338.